[cfe-commits] [pr12251][patch] Use i1* for boolean loads and stores

[Rafael Espíndola]

Sending to cfe-commits. Sorry for the noise on llvm-commits.

2012/3/13 Rafael Espíndola <rafael.espindola at gmail.com>:
> In the testcase of pr12251 we use a i8* for loading a boolean value:
>
>  %3 = load i8* %2
>  %4 = trunc i8 %3 to i
>
> The problem then is that LLVM has no way of knowing that %3 can only
> be 0 or 1. A C file using uint8_t would have compiled to the same IL.
>
> It would be nice to have a general way of telling LLVM that a value is
> known to be in a range, but for the specific cases of booleans we can
> do better by just using a more restrictive type. This patch changes
> clang to use i1 pointers for doing the loads and stores. This changes
> the generated code from
>
>    movb    (%rdi), %al
>    andb    $1, %al
>    ret
>
> to just
>
>    movb    (%rdi), %al
>    ret
>
> I think the idea can be extended for systems that use 32 bit booleans.
> We would just have to be careful with endianess. I got the idea from
> http://llvm.org/bugs/show_bug.cgi?id=8730#c3.
>
> Is this patch OK?
>
> Cheers,
> Rafael